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Author Topic: Suitable variable resistor  (Read 477 times)

SimonCornes

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Suitable variable resistor
« on: October 18, 2021, 11:36:30 am »

This is a simple question from a simple person! I am going to set up a 3v lighting circuit for a boat. But I want to be able to dim the lights if necessary. So I need a variable resistor/rheostat/trim pot to do it. All LEDs so pretty low current but which rotary control should I buy? A case of too much choice!! Can someone point me in the right direction please?
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canabus

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Re: Suitable variable resistor
« Reply #1 on: October 18, 2021, 12:59:09 pm »

Hi Simon


 A low value variable resistor 1000 ohms or less should do the trick !!!


Canabus
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roycv

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Re: Suitable variable resistor
« Reply #2 on: October 18, 2021, 01:15:31 pm »

Hi this is the same as using a speed controller to control an electric motor. 

Have you got a spare small esc?  If you plug it into the Rx, then esc power input battery (3 volt) and the output leads go to the lighting system and disable the battery eliminator circuit (Red wire removed).  Or use one without a BEC circuit.  If you want to do this manually buy a cheap servo tester <£2 and plug the esc into that.

I have not tried it but it should work.  If you have a high frequency one then there should not be any flicker.  The very smallest esc would work and there are tiny ones at less than £4 on a certain auction site.

As far as using a variable resistor goes can you measure the current in the lighting system?  If so the variable resistor should be of the right wattage. But at a guess a 250 ohm resistor of say 2 watts is going to be a bit big and might not turn so easily with a servo.
regards
Roy
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SimonCornes

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Re: Suitable variable resistor
« Reply #3 on: October 18, 2021, 01:16:59 pm »

Thank you - 1k or less, thatís understood!
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malcolmfrary

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Re: Suitable variable resistor
« Reply #4 on: October 18, 2021, 06:22:01 pm »

Variable resistors for "electronic" use (carbon track types) are usually very low power.  Wire wound ones handle more power, but the simplest solution is as suggested, use a small cheap ESC. 
Only caveat - the LED or LEDs will need some protection in the form of resistors to limit the current that the ESC is capable of delivering. If a limit resistor is not fitted they give out all of the light that they can ever give in an instant, and die.  Unless the ESC is "forward only", some form of rectifier arrangement will be needed.  LEDs do stand up to a bit of reverse voltage, but not much before they respond by dying quietly. 
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roycv

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Re: Suitable variable resistor
« Reply #5 on: October 18, 2021, 10:49:07 pm »

Hi having thought a bit more a forward operating diode connected to the start of the LED circuit should remove any possible reverse power.  I presumed the 3 volt supply might be a dry battery?
Roy
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SimonCornes

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Re: Suitable variable resistor
« Reply #6 on: October 18, 2021, 10:54:26 pm »

Hi Roy, yes a twin cell drycell pack, nothing too clever, I toyed with a voltage regulator connected to the 12v gel cell pack but having a stand alone pack simplifies it. There shouldnít be shy trouble with polarity - long LED is positive so I should be okay re polarity.
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roycv

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Re: Suitable variable resistor
« Reply #7 on: October 18, 2021, 11:02:02 pm »

Hi Simon the problem referred to is that the esc can also reverse the current as it would an electric motor.  A single diode in the circuit would mean the led lights would not be able to have a reverse voltage fed to them.
It is unusual to want to dim the leds, another simple idea would be to have an off and then tap the battery for 1.5 volts and then for 3 volts.  You could do this with a servo and 2 microswtches.  Swap the cells about after a while to even up the discharge.

Roy
 
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SimonCornes

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Re: Suitable variable resistor
« Reply #8 on: October 18, 2021, 11:25:35 pm »

Fair point Roy but I will set the light last manually with 3 toggle switches under the forward superstructure- very simple!
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roycv

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Re: Suitable variable resistor
« Reply #9 on: October 18, 2021, 11:28:00 pm »

Hi Simon when we get asked these questions we presume that you are looking for a radio assisted answer.  If you just wished to have some switches its a no-brainer.
Good luck hope it works for you.
Regards
Roy
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roycv

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Re: Suitable variable resistor
« Reply #10 on: October 19, 2021, 07:58:28 am »

Hello Simon if you want simple, thats me, then you need just one switch.  If you use a sigle pole double throw centre off switch then up can be  low, centre is off and down is full.
The leds will connect to one end of the battery and then centre tap the pair of batteries for one pole and go to the full 3 volts for the other.
Roy
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roycv

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Re: Suitable variable resistor
« Reply #11 on: October 19, 2021, 08:26:05 am »

Hi Simon if the last thread idea does not give enough light on just 1.5 volts then do not centre tap the battery but put a 100 ohm resistor to this point and connect the other end to the 3 volts.
Roy
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malcolmfrary

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Re: Suitable variable resistor
« Reply #12 on: October 19, 2021, 09:02:04 am »

Generally, LEDs need 1.9 volts for themselves.  Anything less, nothing happens, any more and they take whatever the supply can deliver, trying to regulate the supply voltage down to 1.9 or whatever they are designed to.  If there is not some means of limiting the current, the LED is destroyed.  Some circuits appear to not have any limiting, but rely on the internal resistance of the battery - keyring LED torches spring to mind.


A handy trick for those with poor memories is to connect two in parallel, one facing each way, with a common resistor, to the battery.  One will light up and protect the other, acting as a reminder and guide.
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SimonCornes

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Re: Suitable variable resistor
« Reply #13 on: October 19, 2021, 09:04:30 am »

Thanks guys. Iíve ordered a 1k variable resistor which I will wire in line from the battery pack and then just turn down the light from 100% to whatever looks right. Canít get much simpler than that! Iím only trying to reflect the intensity of the 1930ís tungsten bulbs so it shouldnít be rocket science!
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Colin Bishop

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Re: Suitable variable resistor
« Reply #14 on: October 19, 2021, 09:58:45 am »

Hi Simon,

You can buy warm white LEDs which mimic tungsten. I used them on my fishery cruiser. As you can see, the masthead lights and searchlight are bright white while the deck and interior  lights have an orange cast.

Colin
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SimonCornes

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Re: Suitable variable resistor
« Reply #15 on: October 20, 2021, 10:34:29 am »

Thanks Colin
I have some 'warm' white LEDs already so I'll see how well they work. I'm still tempted to pick up some tungsten grain of wheat (or grain of rice) bulbs but I appreciate that they are more prone to expiring!
Cheers
Simon
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Colin Bishop

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Re: Suitable variable resistor
« Reply #16 on: October 20, 2021, 11:40:06 am »

Simon,

I also use the GoW/GoR bulbs and run them on reduced voltage which extends their life considerably.

Many models tend to have lights which are far too bright compared to the full size ones.

Colin
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SimonCornes

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Re: Suitable variable resistor
« Reply #17 on: October 20, 2021, 11:48:55 am »

I'm in the same camp as you - its okay to have very bright lights on things like modern fishing boats and oil rig supply vessels where the lights are intense but you have to look at your model in context and back in there 1920's and 30's bulbs were pretty 'yellow' and generators didn't have great outputs so 'dimmer' is always better than 'brighter' for scale effect. I have a customs/excise cutter with GOW bulbs in the compass binnacle, engine/boiler room, forward cabin, mast head and navigation lights and they work very well but changing the one in the compass binnacle would not be easy!
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